Earth breathes in and out, murmuring gently to itself as it does so. The habit has been ascribed to the tickling effects of ocean waves — but a new-found twisting oscillation might reopen the search for the source.
Over the past decade, the word 'hum' has acquired a special meaning for seismologists. No longer just what they might do under the shower, it connotes for them a fundamental resonant oscillation of the Earth. A sequence of these oscillation modes, with periods of between around 2 and 5 minutes, was first identified1,2,3 in 1998. These were all 'spheroidal' modes, representing perturbations of the planet's equilibrium surface, rather akin to the effect of waves on water. Writing in Geophysical Research Letters4, Kurrle and Widmer-Schnidrig now introduce a further, entirely different mode — 'toroidal' hum, in which parts of Earth's surface twist around in the horizontal plane (Fig. 1).
The existence of this low-frequency Earth hum is not the surprising thing. Seismic noise is ubiquitous, generated by various natural processes such as falling water (the impact of, say, the Niagara Falls is not confined to the surface) and even swaying trees, as well as all manner of human activities. It is the magnitude of the hum that is disconcerting5,6: its summed amplitude is equivalent to a continuous earthquake of magnitude 6. (Because the waves are at such a low frequency, we humans cannot sense them; as they represent no threat to our well-being, there has presumably never been a need to evolve such a capability.) An earthquake of this size occurs once every three days on average; clearly, seismic activity cannot sustain hum of such magnitude and continuity.
Since those first intriguing findings, the ocean has by general consensus been identified as the most likely source of Earth hum: the origin of the excitations seems to lie in oceanic areas at mid-latitudes, between about 30° and 60° north and south7,8. In addition, the amplitude of the effect has a periodicity of six months, with a maximum occurring in each hemisphere during its winter; satellite data show that ocean waves are particularly large at mid-latitudes during the winter months.
The proposal9,10, which borrows an idea of some 60 years ago11, is that so-called infragravity waves, which are known to have the same sort of periods as the hum8,12, transmit this oceanic motion to the solid Earth. These waves are similar to tsunami waves — low-frequency, long-wavelength ocean waves that move the whole column of ocean waters, from surface to sea floor, as they propagate. The collision of such waves could produce large pressure variations11, and thus excite the hum. A problem is that infragravity waves are mainly known to be a phenomenon of shallow water, although a mechanism for generating them in the deep ocean has recently been proposed13.
Even so, a direct interaction between the atmosphere and the solid Earth has not been ruled out as a source of the hum. Atmospheric and oceanic effects are difficult to separate: when we see large-amplitude ocean waves, the cause is likely to be an atmospheric effect, namely strong winds. The observant frequent flyer from New York to Paris or Tokyo to San Francisco will note that, during winter in the Northern Hemisphere, flights are often diverted from the shortest geographical route, a great circle over the Arctic, to a more southerly route of near-constant mid-latitude. The reason is the saving of one to two hours in flight-time thanks to strong westerly tail winds over the northern Pacific and Atlantic. On those occasions, the same watchful traveller might also, on looking out of the plane window, see rampant ocean waves far below.
Kurrle and Widmer-Schnidrig's analysis of Earth hum, with their discovery of toroidal modes4, brings a new angle to these considerations of oceanic and atmospheric effects. All past work on hum has focused on spheroidal modes: measuring these modes is simpler, because one needs just a single instrument that measures seismic activity in the vertical plane. Twisting toroidal modes, on the other hand, require the analysis of seismograms in the two horizontal dimensions of Earth's surface. The interpretation of these seismograms is further complicated by the coexistence of spheroidal and toroidal modes in them, as well as noise generated by the local tilt of geological strata just under Earth's surface.
But the authors now show that there are peaks in the oscillation-frequency spectrum that correspond exactly to predicted frequencies of toroidal modes14. Whereas it is easy to see how the broadly analogous up-and-down action of ocean waves might produce spheroidal oscillation modes, it is less easy to see how oceanic infragravity waves might generate toroidal modes.
So what are the alternative explanations? Atmospheric theories developed to explain hum excitation in the past 10 years6,15 have considered only the role of local variations in atmospheric pressure, which impinges vertically downwards at each point on Earth's surface. Again, this vertical force might help to explain the spheroidal modes, but it is irrelevant to the excitation of toroidal modes. Fresh thinking is thus required, whether the source for the newly discovered modes lies in the atmosphere or in the oceans. One might speculate on possible mechanisms: perhaps winds exert shearing forces on the solid Earth through topographic coupling — when an air mass hits a mountain range, for instance — or perhaps long-period ocean waves hitting the undersea walls at continental shelves are generating horizontal forces.
Seismic noise is indeed ubiquitous, but, as Kurrle and Widmer-Schnidrig show4, it also crops up in different forms. By learning more about the hierarchy of mechanical forces that act among the atmosphere, the oceans and the solid Earth, we might hope to become wiser about the origin of Earth's fundamental hum.
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